1. Field of the Invention
The present invention relates to a liquid crystal display device in which alignment films are formed on the surfaces of substrates, and a method for manufacturing the same, and more particularly relates to a liquid crystal display device in which thin film transistors are formed as switching elements, and a method for manufacturing the same.
2. Description of the Related Art
In a general liquid crystal display device, a voltage is applied to a liquid crystal layer sealed between a pair of substrates disposed facing each other, and images are displayed by varying the transmissivity of the light passing through this liquid crystal layer so that the relative brightness is controlled. For example, in the case of a liquid crystal display device using an active matrix driving system, a TFT substrate on which TFTs (thin film transistors) are formed and a counter substrate on which a transparent electrode is formed are disposed so that the surface on which the TFTs are formed and the surface on which the transparent electrode is formed face each other, and these parts are connected to each other via a conductive material. Furthermore, the TFT substrate and counter substrate are bonded to each other by a sealing member which is formed so that this sealing member surrounds the display region, and the liquid crystal layer is sealed in a space that is formed by the TFT substrate, counter substrate and sealing member. Moreover, alignment films that are used to align the orientation direction of the liquid crystal in the liquid crystal layer in a fixed direction are respectively formed on the surfaces of the respective substrates. Generally, the method that is used to form these alignment films is a flexo printing method in which an alignment plate on which protruding parts are formed on the portions where alignment films are to be formed is coated with an alignment agent, and this alignment agent is transferred to the respective substrates by printing.
In recent years, furthermore, there has been an increased demand for compact liquid crystal display devices, and at the same time, there has been a demand for smaller frame portions surrounding the image regions, i.e., a demand for narrower frames. One method for achieving narrower frames in liquid crystal display devices is a method in which an electrical conductivity electrode (transfer electrode) which is used to obtain electrical conductivity between the substrates is mounted in a region in contact with the sealing member on the surface of the TFT substrate, conductive particles are added to the sealing agent that forms the sealing member, and the TFT substrate and counter substrate are electrically connected to each other via the transfer electrode and sealing member. As a result, substrate connecting parts composed of a conductive material or the like mounted further to the outside than the sealing member become unnecessary; accordingly, the frame of the liquid crystal display device can be made narrower. However, in cases where such a structure is used for the liquid crystal display device, the formation of an alignment film on the surface of the transfer electrode must be avoided in order to ensure electrical conductivity with the sealing member. Accordingly, in cases where the alignment films are formed by printing, it is necessary to form a fixed gap between the peripheral edge of the display region and the inside edge of the sealing member in accordance with the printing precision of the alignment film. Furthermore, the alignment plate precision, printing position precision, and the like may be cited as factors that determine the printing precision of the alignment film.
In the past, therefore, in order to solve these problems, a method has been proposed in which an alignment film is formed over the entire surface of the substrate, and specified portions of this alignment film are then removed as necessary (for example, see Japanese Laid-Open Patent Application Nos. 2001-183690, 11-2820, and 2000-47241). FIG. 19 is a sectional view showing the electrically conducting part of the liquid crystal display device in Japanese Laid-Open Patent Application No. 2001-183690 (Prior Art 1). In the liquid crystal display device 100 of Japanese Laid-Open Patent Application No. 2001-183690, external wiring 105 formed on a transparent substrate 101 and internal wiring 107 formed on a transparent substrate 102 are connected to each other by conductive particles 104 included in a sealing member 103, as shown in FIG. 19. These conductive particles 104 are particles in which a conductive layer 104b composed of a metal is formed on the surfaces of plastic particles 104a. Furthermore, in the liquid crystal display device 100, respective alignment films 106 and 108 are formed on the external wiring 105 and internal wiring 107, and the conductive particles 104 break through these alignment films 106 and 108 and come into contact with the external wiring 105 and internal wiring 107.
Thus, in the liquid crystal display device 100 of Japanese Laid-Open Patent Application No. 2001-183690, the conductive particles 103 break through the alignment films 106 and 108 and cause electrical conductivity of the external wiring 105 and internal wiring 107. There is therefore no need to avoid the electrically conducting parts in forming the alignment films. As a result, there are no restrictions on the printing positions of the alignment films, so that the frame of the liquid crystal display device can be narrowed. Furthermore, Japanese Laid-Open Patent Application No. 2001-183690 also discloses a method in which alignment films 106 and 108 are formed over the entire surface, and the alignment films in the electrically continuous portions are then removed by plasma ashing or the like.
FIG. 20 is a sectional view showing the electrically conducting part of the liquid crystal display device described in Japanese Laid-Open Patent Application No. 11-2820 (Prior Art 2). In the liquid crystal display device 110 of Japanese Laid-Open Patent Application No. 11-2820, a TFT substrate 111 in which an external connection terminal 113 is formed on one surface, and a counter substrate 112 in which a transparent electrode 114 is formed on one surface, are disposed so that the surfaces on which the external connection terminal 113 and transparent electrode 114 are formed face each other, and a liquid crystal layer 120 is sealed between these substrates, as is shown in FIG. 20. Furthermore, a plurality of inter-cell spacers 121 are disposed between the TFT substrate 111 and the counter substrate 112, and a sealing member 117 is formed around the periphery of the TFT substrate 111 and counter substrate 112. Moreover, in the TFT substrate 111 and counter substrate 112, alignment films 115 and 116 are respectively formed over the entire surface of each substrate, and conductive spacers 119 dispersed in an epoxy resin 118 break through the alignment films 115 and 116, so that the external connection terminal 113 and transparent electrode 114 are connected to each other. In this liquid crystal display device 110, the alignment films 115 and 116 are pierced by the conductive spacers 119 which are harder than the alignment films 115 and 116, and which have projections on the periphery. The external connection terminal 113 and the transparent electrode 114 are electrically connected, making patterning of the alignment films unnecessary and dispensing with the design margin of the printing plate so that the display area can be enlarged.
FIG. 21 is a sectional view showing the electrically conducting parts of the liquid crystal display device described in Japanese Laid-Open Patent Application No. 2000-47241 (Prior Art 3). In the liquid crystal display device 130 of Japanese Laid-Open Patent Application No. 2000-47241, as is shown in FIG. 21, a TFT substrate and a counter substrate are disposed facing each other, and a liquid crystal layer 144 is sealed between these substrates. Furthermore, a plurality of inter-cell spacers 143 are disposed between the TFT substrate and counter substrate, and a sealing member 142 is formed around the periphery of the TFT substrate and counter substrate. In the TFT substrate in this liquid crystal display device 130, TFTs 133 are formed on top of a transparent substrate 131, and a common terminal 136 is formed on top of an insulating layer 134 that is formed so as to cover the TFTs 133. Pixel electrodes 137 hat are electrically connected with TFTs 133 are formed on top of an insulating layer 135, which itself is formed on top of the insulating layer 134. Furthermore, an alignment film 138 is formed so as to cover portions other than the common terminal 136. Furthermore, in the counter substrate, a common electrode 139 is formed on top of a transparent substrate 132, and an alignment film 140 is formed so as to cover portions other than the electrically conducting parts. Moreover, the common terminal 136 and common electrode 139 are connected to each other by a conductive resin 141. In this liquid crystal display device 130, the alignment films 138 and 140 are formed on the entire surfaces of the respective substrates by a rotary coating method. The region immediately above the common terminal 136 on the surface of the alignment film 138 is then coated with the conductive resin 141, the TFT substrate and the counter substrate are pasted together, and the common terminal 136 and common electrode 139 are irradiated with an Nd-YAG laser so that the alignment films 138 and 140 on these parts are melted and removed.
Disclosures also exist (for example, see Japanese Laid-Open Patent Application Nos. 10-268310, 2000-47228, and 2003-295191) regarding liquid crystal display devices in which a portion of the sealing member is formed on an alignment film in order to prevent thickness defects from being caused in display cells by irregular thickness of the alignment films and to prevent the alignment films and sealing member from peeling. FIG. 22A is a plan view showing the display panel of the liquid crystal display device described in Japanese Laid-Open Patent Application No. 2000-47228 (Prior Art 4). FIG. 22B is a sectional view along line A-A in FIG. 22A. The display panel 150 of the liquid crystal display device of Japanese Laid-Open Patent Application No. 2000-47228 has a lower substrate in which a transparent electrode 153 is formed on a transparent substrate 151, and in which an alignment film 155 is formed so as to cover this transparent electrode 153. The panel further has an upper substrate in which a transparent electrode 154 is formed on a transparent substrate 152, and in which an alignment film 156 is formed so as to cover this transparent electrode 154. The two substrates are disposed so that the surfaces on which the alignment films 155 and 156 are formed face each other, as is shown in FIGS. 22A and 22B. Further disposed between the upper substrate and lower substrate are a plurality of spacers 158a and a sealing member 157 that is formed so as to surround the display region and has a liquid crystal injection port 159 formed in one part. The gap between the upper and lower substrates is maintained at a constant value by the sealing member 157 and spacers 158. Furthermore, a liquid crystal layer 160 is sealed inside the part surrounded by the sealing member 157 between the alignment film 155 and alignment film 156. In this liquid crystal display panel 150, the sealing member 157 is disposed straddling regions in which the alignment films 155 and 156 are formed and regions in which these alignment films are not formed. Generation of gas bubbles when the liquid crystal is injected can therefore be prevented, and the bonding strength of the sealing member can be improved. Furthermore, in the liquid crystal display device described in Japanese Laid-Open Patent Application No. 2003-295191 (Prior Art 5), the sides parallel to the rubbing direction in the alignment films are formed in a non-linear shape such as a buckled shape, sawtooth shape, wave shape, C shape or the like.
However, the following problems have been encountered in the abovementioned prior art. Specifically, the following problem arises in the case of methods in which electrical conductivity of the upper and lower substrates is obtained by piercing the alignment films by means of conductive particles as in the liquid crystal display devices of Japanese Laid-Open Patent Application Nos. 2001-183690 and 11-2820. Namely, a sufficient electrical conductivity cannot be obtained using the fine electroconductive particles that are currently in common use and are obtained by forming a metal thin film on the surfaces of plastic particles. The following Table 1 shows the electrical conductivity resistance of the upper and lower substrates with respect to the overlap pressure of the upper and lower substrates when an alignment film was present on the transfer electrode and when no alignment film was present on the transfer electrode in a case where the liquid crystal display device of Japanese Laid-Open Patent Application No. 2001-183690 was manufactured by forming a polyimide thin film with a pencil hardness of 2H as an alignment film. The material for the film was prepared by a method in which conductive fine particles formed by coating the surfaces of plastic fine particles, which were primarily composed of diphenyl benzene and had a particle size of 5 μm, with a nickel-gold alloy were added to the sealing agent at the rate of 0.1 mass %.
TABLE 1Vertical circuit resistance (Ω)Overlap pressureWithout alignment(kPa)With alignment filmfilm30In excess of 2 × 1077.345In excess of 2 × 1077.680In excess of 2 × 1077.4
As is shown in the abovementioned Table 1, in cases where conductive particles obtained by forming a metal thin film on the surfaces of plastic particles are used, forming an alignment film on the transfer electrode makes it difficult to achieve electrical conductivity between the upper and lower substrates even if the overlap pressure is increased to a high value. This problem can be solved by using high-hardness conductive particles such as metal particles or the like. However, in cases where high-hardness conductive particles are used, forming a flattening film composed of an organic film on the TFT substrate in order to alleviate the step differences causes the flattening film to be dented by the conductive particles so that the amount of deformation increases, thus making it impossible to obtain a uniform cell gap. Furthermore, depending on the overlap pressure, there is a danger that this may lead to destruction of the ITO (indium tin oxide) film that constitutes the transfer electrode.
Furthermore, a method in which an alignment film is formed over the entire surface, and the alignment film on the transfer electrode is then removed by plasma ashing, has been proposed in Japanese Laid-Open Patent Application No. 2001-183690 as a means of solving the abovementioned problem. In this case, however, the productivity drops and the cost is increased as a result of the need to prepare a mask to protect the display region, the need for a positioning mechanism to position the mask, and the addition of an ashing process. The following problem also arises in cases where the abovementioned flattening film composed of an organic film is formed. Namely, the flattening film is also removed by this ashing.
Furthermore, the liquid crystal display device 130 of Japanese Laid-Open Patent Application No. 2000-47241 is a conventional liquid crystal display device in which an electrically conducting part composed of a conductive resin 141 is mounted on the outside of the sealing member 142. However, there are cases where a method for removing the alignment film of the electrically conducting part by melting with a laser is applied to a narrow-frame liquid crystal display device in which a transfer electrode is mounted on the TFT substrate and in which electrical conductivity of the counter substrate and TFT substrate is obtained via this transfer electrode and via a sealing member that includes conductive particles. In such cases, the heat resistance of the sealing member is generally inferior to that of the alignment film, causing the sealing member to be melted by the laser. Another problem is that particles of the sealing member, the alignment film, or the transfer electrode scatter around and remain inside the cell, producing display defects.
Furthermore, in a narrow-frame liquid crystal display device in which the counter substrate and TFT substrate are made electrically continuous via the abovementioned transfer electrode and the sealing member that includes conductive particles, there are cases where the sealing member is disposed so as to straddle regions in which an alignment film is formed and regions in which an alignment film is not formed, and a transfer electrode is mounted in a region on the surface of the TFT substrate that is in contact with the sealing member, as in the conventional liquid crystal display devices disclosed in Japanese Laid-Open Patent Application Nos. 2000-47228, 2003-295191, and 10-268130. In such cases, electrical conductivity is obtained between the counter substrate and TFT substrate, but if a flattening film composed of an organic film is formed on the TFT substrate, the surface roughness of the transfer electrode is decreased, and the adhesive strength of the sealing member drops.
FIGS. 23A and 23B are sectional views showing the method used to test the adhesive strength of the sealing member in the order of the processes involved. As is shown in FIG. 23A, the following elements are bonded by a sealing member 167 that includes conductive particles and a gap material: a substrate in which an inter-layer insulating film 163 whose uppermost layer is an organic flattening film is formed on top of a glass substrate 161 so as to cover the electrode layer 162, a transfer electrode 164 that consists of ITO and is connected to the electrode layer 162 via contact holes is formed on top of the inter-layer insulating layer 163, and a substrate in which a counter electrode 166 composed of ITO is formed on top of a glass substrate 165. The transfer electrode 164 and the counter electrode 166 are connected to each other via this sealing member 167, after which these two substrates are subjected to a tension test. As a result, interfacial peeling occurs only in portions of the transfer electrode 164 formed on top of the inter-layer insulating film 163, and destruction occurs inside the sealing member 167 in other portions, as is shown in FIG. 23B.
One of the reasons for this is the difference in the surface roughness of the respective electrodes. Table 2 below shows the surface roughness of the region A of the transfer electrode 164 that is formed on top of the inter-layer insulating film 163, the surface roughness of the region B of the transfer electrode 164 that is formed on top of the electrode layer 162, and the surface roughness of the region C of the counter electrode 166. Furthermore, the surface roughness values shown in the following Table 2 are values obtained by standardizing results measured by means of an AFM (atomic force microscope), with the value for region A taken as 100. The adhesive strength values for the respective regions are also shown in the following Table 2. The adhesive strength values indicated in the following Table 2 are shown as x in cases where interfacial peeling occurred, and are shown as O in cases where internal destruction occurred.
TABLE 2Surface roughnessMeasurement regionAdhesive strengthRa (relative value)AX100B◯324C◯146
As is shown in the above Table 2, the portions of the transfer electrode 164 formed on top of the inter-layer insulating film 163, i.e., the portions formed on top of an organic flattening film, shows a small surface roughness, and therefore shows a low adhesive strength. Accordingly, in order to improve the adhesive strength of the sealing member of the liquid crystal display device, it is advisable to minimize the size of the transfer electrode formed on top of the organic flattening layer. However, considering the printing precision of the alignment films, it is difficult to achieve any further reduction in the size of the electrode regions.